What is the new tab URL (browser.newtab.url)?
It is probably possible to set a background image via code in userContent.css<br />
This is an example for the about:home page, place the image in the chrome folder along with the userContent.css file.
Add code to the userContent.css file.
*http://kb.mozillazine.org/userContent.css
The customization files userChrome.css (user interface) and userContent.css (websites) are located in the chrome folder in the Firefox profile folder.
*http://kb.mozillazine.org/Editing_configuration
<pre><nowiki>/* about:home */
@-moz-document url(about:home){
body{ background: url("abouthome.jpg") rgb(180, 218, 244) center no-repeat !important; }
#snippetContainer { display: none !important; } /* hide the snippet container */
}</nowiki></pre>